In semiconductor fabricating lines, keeping a high level of cleanliness is necessary in order to maintain clean rooms or process chambers. Accordingly, it is very important to quickly detect even small amounts of airborne molecular contaminants. A variety of monitoring systems have been proposed for monitoring airborne molecular contaminants (AMCs), such as acetone, ethanol, benzene, toluene or dichloroethane, in a semiconductor fabricating apparatus. Unfortunately, these monitoring systems are high-priced and require the passage of a considerable amount of time for analyzing the AMCs which are present.
A typical apparatus for detecting the presence of undesirable substances is a surface acoustic wave gas sensor. The surface acoustic wave gas sensor includes a piezoelectric substrate, an input transducer, an output transducer, and a sensitive film. An electrical signal applied to the input transducer is converted into a wave by the piezoelectric substrate. The signal is transmitted to the output transducer through the piezoelectric substrate acting as a transmission medium. The wave reaching the output transducer is converted into an electrical signal by the piezoelectric effect. When a substance to be detected is absorbed in the sensitive film, the frequency of the wave is fluctuated. Depending on the fluctuated frequency of the wave, it is possible to determine whether there is an undesirable substance present, as well as its concentration.
According to the substances to be detected, the surface acoustic wave gas sensor uses a sensitive film coated with a sensitive substance which can readily absorb an undesirable substance to be detected. The sensitive film must have a high sensitivity so as to be responsive to the presence of the substance, i.e., exhibit a low detection limit. Further, the sensitive film must retain its high sensitivity property relative to the gas to be detected, and it must detect the gas as soon as possible.
Presently, there is no substance which is sensitive to a material such as acetone. Although there are sensitive substances which are sensitive to ethanol, benzene, toluene, and dichloroethane, their sensitivities are low. Therefore, it takes a considerably long time to detect the presence and the amount of these substances.